Why the heart talks to the brain
Science is still learning about the neurophysiology of stress and trauma. Along that journey of discovery science discovered that it's "not all about the brain". In fact the heart, for example, communicates with and influences the brain via the nervous system, hormonal system and other pathways.
It's fundamentally a matter of survival and the body seeks balance (homeostasis) at all levels down to the single cell. The brain actually hosts conversations and it is the whole body that thinks - not just the brain.
Historically, mankind viewed the mind and body as separate entities and some cultural norms were keen to elevate the mind as superior and the body as something that was of lower importance. These days we should know better.
Whilst the communication between the heart and brain is perhaps not recognised by the general public, Charles Darwin way back in 1872 understood that this two way communication took place. A hundred years ago, the Autonomic Nervous System (ANS) was described and only in the 1950's was it noted that actually we had an ability to consciously influence the ANS - it wasn't all beyond our influence and on "automatic pilot"
One of the more contemporary theories of the impact of emotion on the body-mind system came from Stephen Porges' work over three decades. His ideas are founded on the deep interrelationship of the cranial and vagal nerves and our two vagal motor systems. He suggests that part of this system developed more recently (in evolutionary terms) along with something called the HPA axis. The hypothalamic pituitary adrenal (HPA) axis is often described as our central stress response system.
Porges supposed that only one of the two vagus branches responsible for "fight, flight or freeze" developed myelination. The newer, myelinated branch, involves immediate and persistent effects such as heart rate increase, pupil dilation, decreased digestion, sweaty palms etc. This branch evolved in mammals and other vertebrates. The evolutionary ancient, unmyelinated branch invokes reduced metabolism and immobilsation to "act dead" or hide. Thus defence as far as our nervous system is concerned is either the very rapid and precise fight or flight or the much slower immobilisation.
In early stages of trauma it's the myelinated response that occurs and in the more advanced stages it is the unmyelinated responses take effect. In depression for example, as the patient loses affect, it is this unmyelinated response at work. It's as if its the body's last gasp attempt at finding balance when the fast response, myelinated system fails.
Porges demonstrated the two-way communication taking place between the brain and heart. The brain stem, where the cranial nerves mitigate their inputs, is involved in auditory/visual actions through networks linking it and frontal cortical areas. In general tems, neurofeedback involves audiotry and visual networks and may strengthen the interrelationships within the brainstem and between the brainstem and other cranial nerves including the vagal nerves.